The detection of occult blood in the stool is a recognized method for the early diagnosis of carcinomas of the colon. The chemical detection of occult blood in the stool is an old, tried and simple method of investigation. In this case, the principle of the test if based on the peroxidase action of hemoglobin, whereby, in the presence of hydrogen peroxide, an indicator such as benzidine, o-toluidine or guaiacum is oxidized and a blue coloration results. This is, of course, a non-specific reaction, since besides hemoglobin other stool components such as food, primarily blood sausage, red meat as well as vegetable constituents, and intestinal bacteria also have peroxidase activity.
There are, for example, commercialized tests available for the detection of occult blood in the stool. An example of such test is the "Colo-Rectal-Roche-Test.RTM." made available by Hoffmann-La Roche AG Diagnostica. This test is available in the form of test envelops having defined test fields to which a sample of stool is added. The paper covering the test field is impregnated with guaiacum which acts as redox indicator. The application of an ethanol hydrogen peroxide solution will dissolve the water insoluble guaiacum and start the peroxidation reaction with the hemoglobin of the sample to provide a blue color.
In the commercialized tests for the detection of occult blood in the stool, which are available in the form of test envelopes, the patient is requested as a precaution to give up the consumption of raw and semi-raw meat as well as of sausages (e.g. tartare, steak, liver, salami, blood sausage) for three days before the beginning of the test and during the period of the test.
This and other precautions are troublesome for the patients and are not entirely capable of excluding false positive results, which necessitate time-consuming and expensive subsequent investigations such as coloscopy or rectoscopy.
For this reason it has also been proposed (Songster, C. L. et al. in American Cancer Society 1099 et seq., 1980), in the case of a conventional test envelope impregnated with guaiacum, to use an additional stool sample reception site which is not impregnated with guaiacum and, in the case of a positive guaiacum sample, to investigate the content of this additional stool sample reception site for the presence of human hemoglobin with the aid of an immunological method.
This procedure has a disadvantage in that this additional stool sample is not absolutely identical with the stool samples which are subjected to the guaiacum test; for example, the additional stool sample has been removed from another stool evacuation or from another part of the stool, which again can lead to uncertainties.
There accordingly exists a need to investigate stool samples, which have been subjected to the peroxidase test, for the presence of components of human origin which are not normally present in human stool and which would interfere with an accurate determination of the occult blood by the peroxidase test.
Since the peroxidative test or method is carried out with the aid of an ethanolic solution, there has hitherto been the opinion that this method destroys the immunological properties of components of human origin which are not normally present in human stool.